This invention relates generally to channel selector systems utilized in tuning a television receiver to a desired channel and specifically to channel selectors having a single rotatable shaft by which both UHF and VHF channels can be selected in a comparable manner.
Commercial television stations broadcast in two frequency spectra, the very high frequency (VHF) and the ultra high frequency (UHF) bands. The VHF band includes channels 2 through 13 operating at frequencies of from 57 megahertz (MHz) to 213 MHz while the UHF band includes channels 14 through 83 operating over a frequency range of from 473 MHz to 887 MHz. The VHF broadcast band is generally further broken down into low VHF for channels 2 through 6 operating at frequencies from 57 MHz to 85 MHz and high VHF for channels 7 through 13 operating at frequencies from 177 MHz to 213 MHz. Early dual band tuners for television receivers included separate controls and circuitry for the UHF and VHF frequency bands. These initial tuners tended to be physically large, overly complex and included a large number of components which resulted in low reliablility and high unit cost. The development of the variable capacitance diode, or varactor diode, however, and its incorporation in the variable tuning stages of a television receiver tuner significantly alleviated the aforementioned limitations of the primarily electro-mechanical tuners of the prior art.
In general, a television tuner has several tunable circuits which can be tuned to the frequency of the channel to be received and an oscillator tuner circuit which is offset from the tunable circuit frequency by the frequency of the intermediate frequency amplifier. It is this intermediate frequency signal which is actually used in tuning to the desired channel but the generation of this intermediate frequency signal is extremely important in all aspects of tuner performance. Varactor diodes are used in the generation of the intermediate frequency signal for varying the resonant frequency of the tunable circuit. Tuning is accomplished by varying a DC voltage applied to the varactor generally by means of a variable resistor such as a potentiometer, which results in a variation in capacitance of the varactor. By thus varying the capacitance of the tuning circuit, its resonant frequency may be accurately changed in order to produce the desired intermediate frequency signal when mixed with the output of the variable frequency local oscillator stage.
The replacement of the earlier electromechanical tuners with the electronic tuner made possible by the use of varactor diodes was accompanied by the development of a single rotary-type control, combination UHF-VHF tuner system which replaced the earlier systems having separate UHF and VHF tuner controls. These earlier systems included a separate control knob or dial for the two tuners with a 13-position detent-type tuner for VHF channel selection and for UHF band selection. With the UHF band selected on the VHF tuner, UHF channel selection was available by means of a separate control for the UHF tuner. While VHF tuning was accomplished by a detent-type tuner, UHF tuning was performed by continuously tuning through all frequencies in the UHF spectrum. While a detent arrangement over both frequency bands may be desirable, it is not practical in view of the large number of channels (70) available in the UHF broadcast band. On the other hand, the mechanical action of a detent tuning system results in reduced tuning system reliablilty and operating lifetime. In addition to these shortcomings, the earlier combination UHF-VHF tuners failed to comply with Federal Communications Commission regulations requiring that the tuning arrangement for a domestic television receiver must be so designed as to enable the viewer to select, with equal ease and in a comparable manner, all UHF and VHF channels.
One example of a prior art UHF-VHF combination tuner is disclosed in U.S. Pat. No. 3,707,877 wherein is described an all-channel tuner controlled by a single rotatable selector shaft which is axially movable. Axial movement of the selector shaft permits engagement of appropriate gears in selecting either the UHF or VHF broadcast band. This action also results in the engagement of appropriate gears for channel selection. A DC voltage is provided to varactor diodes in the tuner by means of an infinitely variable resistor resulting in a continuous, nondetent, tuning system for both UHF and VHF bands. However, selection of UHF and VHF channels is not comparable since only one revolution of the control shaft is required to tune from channel 14 to channel 83 while two revolutions of the control shaft are required in tuning from channel 2 to channel 13. Thus, the inter-channel separation between UHF channels is one-half the frequency range of VHF inter-channel separation. This results in increased difficulty and reduced accuracy in UHF tuning. In addition, the utilization of a single variable resistor requires the use of a selectively engageable gear system which significantly increases the complexity of this all-channel tuning system while reducing its reliability because of the high number of moving parts. Another multiband tuner control system is disclosed in U.S. Pat. No. 3,942,122 wherein is described a channel selection circuit including a pair of switch bands, one of which is used to select the tens digit of a television station number and the other of which is used to select the units digit. Thus, the bank of the units switch is used to select one of ten UHF potentiometers in a group used to tune to each of the ten channels which are possible within each UHF decade. This large number of adjustable potentiometers, in one embodiment of the invention described therein 60 potentiometers are shown, necessarily results in channel tuning inaccuracies primarily due to contact resistance variations within the potentiometers. Such tuning inaccuracies are particularly critical in the UHF band where inter-channel frequency spacing is much smaller than in the VHF band. The complexity of this system outweighs any advantages available from the single control feature or the equivalent tuning procedure for both UHF and VHF channels. Still another all-channel UHF-VHF television tuner is disclosed in U.S. Pat. No. Re. 29,786. This invention, however, utilizes a single detented selector shaft for individually selecting channels in both UHF and VHF bands. It thus lacks the inherent reliability of a continuous-type, nondetent tuning system with its absence of mechanical stresses and high impulse contact forces between components.
In order to accommodate the wide frequency spectrum covering the UHF and VHF television broadcast bands so-called hyper abrupt junction varactor diodes were initially incorporated in all-channel type tuners. These hyper abrupt varactor diodes differ from abrupt varactor diodes in that they exhibit larger capacitance changes for a given tuning voltage change. This permits greater frequency coverage for a particular voltage variation in the tunable stages. This enhanced performance is achieved by increasing the dopant gradient at the junction of the varactor diode. This increased frequency tuning capability in hyper abrupt varactor diodes is not without its shortcomings, however. For example, hyper abrupt varactor are not only more expensive than abrupt varactors but also exhibit a significantly greater variation in performance characteristics. These performance variations become particularly critical in the typical tuner system in which several varactors must be carefully selected to insure matched performance among the various tuning stages in both the UHF and VHF bands. This careful matching procedure further increases the manufacturing costs of hyper abrupt varactor diode tuning systems. These disadvantages tend to outweigh the advantages of increased frequency band coverage and enhanced channel selection separation over both UHF and VHF bands provided by hyper abrupt varactor tuners.
U.S. Pat. No. 3,962,643 discloses an abrupt junction varactor diode television tuner in which a plurality of equally spaced, circumferentially arranged metalized areas form stationary contacts each of which permits tuning to a particular channel. Because of the large number of UHF channels and the fact that each is represented by an individual metal contact position on the circumference of a circle, this tuner system affords tuning to only six UHF channels in combination with the twelve VHF channels. End terminals of each divider network in the individual tuning stages are coupled to adjustable potentiometers for setting the tuning voltage at each extreme of the voltage divider. In this manner correct tuning voltages for signals between these end points are automatically obtained because of the precise ratio between resistors in the voltage divider network and the predictable capacitance-voltage characteristics of abrupt junction varactor diodes. This approach thus fails to offer a continuous-type tuning capability and permits tuning to only a very limited number of UHF channels with the common UHF-VHF rotatable control shaft. See also U.S. Pat. No. 3,965,427.
The present invention, however, overcomes the aforementioned limitations by providing an all-channel selector for controlling a UHF-VHF television tuner in which a substantially linear relationship exists between single control shaft angular rotation and channel number displacement over both tuning bands with continuous, non-detent channel selection available. The instant invention not only fully satisfies FCC regulation regarding equivalence in tuning to UHF and VHF channels, but also operates with abrupt junction varactor diodes while achieving bandwidth tuning and channel number angular separation heretofore available only with hyper abrupt varactor diode tuning systems.